When you install Crysis, it will install to default folders - leave it there as most of the game is 32bit files.
However, when you want to *run* Crysis, click Start -- > Games --> Crysis - and then, if you ALT - TAB over to the desktop after it starts and run task manager, you'll see it auto selects the 64bit game module to run.
However, if you have a separate partition / drive, I *highly* recommend that you use that for game installations - mainly because you'll have less chance of running into game incompatibilities caused by UAC. Even if it means using Vista Disk Partition Manager to resize your current partition, and make a new small (20 GB) partition at the end, you'll be better off than running it out of Program Files / Program Files (X86) folders.